1. Field of the Invention
This invention relates to the main mechanism of a tire vulcanizer, and more particularly to a driving system for the main mechanism.
2. Description of the Prior Art
In a tire vulcanizer, there is employed a main mechanism for performing the shaping and other processing work of green tires. An example of such a main mechanism is disclosed in U.S. Pat. No. 2,775,789 in which an upper ring for holding a bladder such as a rubber bag is constructed in such a way that the bladder may be operably inflated or deflated by vertically displacing a piston rod to which the upper ring is coupled. To move the piston rod up and down, a piston is connected with the piston rod and is housed in a lower portion of a lift cylinder. A fluid is introduced into a fluid chamber provided in the lower portion of the lift cylinder and another fluid chamber formed in the interior of the piston rod through a pipe which communicates with each of the fluid chambers. On the other hand, for shaping and vulcanizing a green tire, high temperature steam is charged into the interior of the bladder through a pipe which communicates the interior and the shaping and curing of the green tire is performed through the bladder.
In a tire vulcanizer equipped with such a main mechanism as referred to in the above, the driving mechanism makes use of a fluid as its driving medium regardless whether it is hydraulic or pneumatic. Thus, a heat loss arises on the lower ring at locations where said lower ring is brought into contact with the bladder, thereby upsetting the heat balance between the upper and lower rings. This in turn deteriorates the uniformity of the vulcanization degree to a certain extent. This drawback may be compensated for by prolonging the curing time in view of the heat loss. However, such a prolonged curing time requires a significant amount of heating medium and results in a wasting of thermal energy.
Where green tires of different sizes are treated by such a tire vulcanizer, it is necessary to adjust the shaping stroke of the vulcanizer. This adjustment is generally carried out by replacement of a spacer provided in the lift cylinder. However, the work associated with such replacement is very time-consuming as the spacer is built in the cylinder. Furthermore, the prior art main mechanism is accompanied by other difficulties in making it water or oil tight and in arranging the piping because it employs a fluid as its driving power source.
Another type of the main mechanism, which employs a mechanical driving mechanism, has also been proposed as a countermeasure to the aforementioned drawbacks of the fluid-driven main mechanism. An example of such a main mechanism is disclosed in U.S. Pat. No. 3,487,507, which incorporates as its constituents a ball screw shaft and a ball nut. The rotary motion of the ball screw shaft is converted to the rectilinear motion of the ball nut, which rectilinear motion inflates or deflates a bladder through a tubular bladder ram connected with the ball nut. However, in this particular vulcanizer, before the green tire is molded under pressure by the upper and lower molds by lowering a socket holding the upper extremity of the bladder by the upper mold after the completion of the shaping of the green tire, the ball screw shaft is rotated by a signal from an electric circuit to lower the tubular bladder ram to a position ready for the pressure molding. This not only makes the main mechanism very complex but also causes the breakage of the main mechanism and leads to serious problems from the viewpoint of safety if the lowering speed and timing of the upper mold are mismatched with those of the lower mold.